1. Field of the Invention
This invention relates to a method of smoothing the surface of a flexible magnetic recording medium. This invention particularly relates to a method of smoothing the surface of a flexible magnetic recording medium wherein the flexible magnetic recording medium is passed through and pressed between at least one set of two rolls at least one of which is a metal roll, thereby to smooth the magnetic surface of the medium.
2. Description of the Prior Art
As techniques for smoothing the surface of a magnetic recording medium, the following methods have heretofore been known widely:
(1) A method wherein the dispersant and the dispersing method used at the step of preparing a magnetic coating solution are improved to form a magnetic layer having a relatively smooth surface at the step immediately after the coating.
(2) A method wherein magnetic layers of recording media after coating and drying are contacted with each other and moved at high speeds with respect to each other to rub, grind and smooth the surfaces of the magnetic layers.
(3) A method wherein the magnetic layer surface is rubbed and ground with fur of animals, plastics, metals, ceramics, or the like.
(4) A method wherein the magnetic layer is smoothed by use of press rolls which are called the supercalender rolls.
However, the conventional methods described above present the problems as described below. Namely, in the method (1), the electromagnetic transducing characteristics, particularly the sensitivity and the signal-to-noise ratio obtained are not satisfactory. In the method (2), the drop out due to chipping of the magnetic layers caused by the grinding is so high that the method cannot be put into practice. In the method (3), it is impossible to conduct the surface smoothing required for a high-density recording medium. In the method (4), surface smoothing is conducted by passing a recording medium several times between a metal roll and a plastic roll. In this method, since the nip pressure of the supercalender rolls is high, large loads are exerted on the rolls and roll noise occurs when the rolls cannot withstand high pressures. Further, the middle portions of the rolls are thermally expanded due to heat generated by the rolls when the rolls are rotated in the pressed condition and/or due to heat for raising the roll temperature to a value within the range of 40.degree. to 80.degree. C. for the purpose of improving the smoothing effect. In this case, the pressing force of the rolls becomes uneven and, therefore, the thickness and/or smoothness of the magnetic recording medium obtained becomes uneven. In the case of a resilient roll, the hardness of the roll becomes uneven or the roll is cracked when the condition as described above continues for long periods.
In the cases of tapes wherein only one tape side is used for recording, for example, video tapes, audio tapes, and computer tapes, the purpose of smoothing the magnetic layer surface can be accomplished by conducting calendering with the magnetic layer surface contacting the surface of the metal roll of the aforesaid supercalender rolls. Therefore, supercalenders, including various improved types, are widely used.
However, since the supercalendering method has various drawbacks as described above, it is necessary to frequently replace the resilient roll or polish the roll surfaces. Thus, the method is troublesome in practical use. This method also has a drawback in that the pressure is too high (the linear pressure is within the range of 200 to 400 kg/cm) and that the roll itself must be heated (to a temperature of up to 80.degree. C.). As magnetic tapes with higher recording density and higher performance come to be required, it is desirable to increase the temperature in smoothing above the working temperature of the conventional supercalender (in order to further smooth the magnetic surface). However, the smoothing temperature cannot be increased as desired for reasons of the conventional apparatus.